1. Field of the Invention
The invention relates to a process for the partial combustion of coal powder, and more specifically, a process wherein a dispersed coal powder is transferred from a fluidization vessel to a combustion reactor.
2. Description of the Prior Art
As will be appreciated by those skilled in the art, the term fluidization designates an expansion of a given amount of powder present in a space under the influence of a gas flowing through that space, which fluidization renders the powder free-flowing. Likewise, the term coal powder designates all finely divided solid fuels that are eligible for partial combustion, such as coal powder, finely divided brown coal, wood dust, etc. Partial combustion is understood to be the combustion of fuel with a smaller amount of oxygen than would be required stoichiometrically for a complete combustion to CO.sub.2 and H.sub.2 O. In addition to oxygen (or air), steam is often introduced into the reactor for partial combustion reactions and is partly converted into hydrogen.
In the partial combustion of solid fuel, preference is usually given to powder, especially coal powder, because of its rapid gasification. The coal powder preference applies both to (1) direct combustion gasification processes, or processes in which the powder and the oxygen are both injected into a reactor to form a flame therein; and to (2) indirect combustion gasification processes, or processes in which the fuel is first gasified at a lower temperature in a fluidized bed or fluidization vessel with the aid of a hot fluidization gas and is subsequently partially combusted in a reactor. In the former type of processes, the pressure in the combustion reactor may unfortunately fluctuate widely as compared to the latter type of processes, which produces numerous instabilities and inefficiencies in the partial combustion process known to those skilled in the art.
In both methods, it is advantageous to carry out the partial combustion at elevated pressure, e.g. at 30 atm or higher. The gasification equipment is then more compact and the product gases become available under high pressure.
According to the indirect method, to which this application usually pertains, the coal powder solids are conveyed from the fluidization vessel to the combustion reactor by passage means while dispersed in a gas, so that theoretically a flow of finely divided coal powder enters the combustion reactor, which is very advantageous for the combustion process proper. In most cases the oxygen-containing and combustion-supporting gas can only be contacted with the coal powder in or near the combustion reactor, because of the ignition hazard outside the reactor. For this reason the fluidization gas used is often an inert gas or at any rate a gas containing little or no oxygen, such as steam, nitrogen, carbon dioxide or product gas.
It was realized by some of those skilled in the art that it would be desirable not only to pressurize the coal powder prior to its introduction into the combustion reactor in a separate fluidization vessel, but (1) to have the pressure in the fluidization vessel be only the smallest increment above that pressure prevailing in the combustion reactor to transfer the dispersed powder to the reactor, and (2) to keep this small necessary increment substantially constant. Hence, even a greater stability in the partial combustion process would occur. If that obstacle would be overcome, it would not be necessary for a process operator to overcome at the same time both (a) a great pressure difference and (b) a great temperature difference between the fluidization vessel and the reactor when the coal powder was introduced into the combustion reactor.
In the past, in attempts to accomplish these objectives, efforts were made by those skilled in the art to discharge the dispersed coal powder from the fluidization vessel in a simple way and with a sufficiently high concentration of solids in the gas, but unfortunately, were not entirely successful. One difficulty was that at the place where coal particles were discharged from the fluidization vessel, the fluidized bed in the fluidization vessel was disturbed so much and obstructed by parts of discharge means in the vessel for flow of the dispersed coal solids to the reactor that the degree of solids loading of the exit gas was low. Still another difficulty was that the vessel discharge dispersions or concentration of particles in any short length of time remained inhomogeneous and irregular. Because of these two disadvantages a relatively larger amount of inert gas was introduced into the combustion reactor than that desired, which resulted occasionally in reactor temperatures becoming too low for effective conversion, and a need for larger size reactors. Also, with inhomogeneous and irregular concentrations of solids in the dispersed coal powder, numerous inefficiencies and instabilities occurred in the reactor known to those skilled in the art.
A method was needed to obviate these drawbacks and provide a process for efficiently discharging the coal powder from the fluidization vessel.